1. Field of the Invention
The present invention relates to a high capacity digital mobile radio system, and more particularly, to a digital mobile radio system which employs the techniques of space diversity and time-division retransmission to form a system where all the required adaptive signal processing is performed at baseband at the base station.
2. Description of the Prior Art
Radio signals are always subject to fading due to natural phenomena, but when one station of a radio link is mobile and moving at variable speeds through various and unpredictable environments, the situation is seriously compounded. In such a situation there are two types of received signal level variations observed. First there is the rapid multipath Rayleigh type fading due to different path cancellations and then there is a slower variation in the mean signal level due to gross path variations from building shadowing and other terrain effects. Both types of signal level variations are functions of the speed of the mobile.
Space diversity has been found to provide one of the best solutions to mobile radio fading. One analog mobile radio system employing space diversity is disclosed in U.S. Pat. No. 3,693,088 issued to A. J. Rustako, Jr., et al on Sept. 19, 1972. There, diversity transmission from the base to the mobile is provided by switching between two spaced base transmitting antennas on command from the mobile. More particularly, means are provided at the mobile station for determining when the signal level then being received by the mobile from a given base station antenna falls below a level which depends upon the nature of the fade itself. When this occurs, the mobile transmits an out of message band signal back to the base which causes the base to switch to a different antenna.
A similarly operated digital mobile radio system is disclosed in U.S. Pat. No. 4,057,758 issued to T. Hattori et al on Nov. 8, 1977. There, a plurality of receiving antenna systems are switched at a constant frequency higher than the signaling rate of the digital baseband signal but less than the frequency shift width of the frequency modulated wave or less than a product of the maximum phase shift of the phase modulated wave and the signaling rate, so that average-power dispersion in a signal element of the digital baseband signal received at the receiving antenna system is effectively compressed. Alternatively, the plurality of antennas may be transmitting antennas which are simultaneously switched to achieve compression of average power dispersion in the baseband signal elements.
The above-described analog and digital systems, however, require the flow of feedback information to control antenna switching, necessitating the use of complex and expensive apparatus at the mobile. Co-phasing of the antenna elements in an analog system has been found to provide transmission from a diversity station by means of a multi-element array, as disclosed in U.S. Pat. No. 3,717,814 issued to M. J. Gans on Feb. 20, 1973. Phase corrected intelligence signals are transmitted from a diversity array transmitter and received in-phase at a monochannel receiver. An individual pilot associated with each diversity branch and frequency separated from the pilots of the other branches is received along with the in-phase intelligence. All of the pilots are fed back, as part of the return modulation, to the diversity transmitter where they are used to establish the proper phase correction for the modulated intelligence transmission. However, the signal processing at the diversity transmitter as taught by Gans occurs at i. f., thus requiring the use of expensive RF hardware to achieve relatively accurate phase correction.
The problem remaining in the prior art is to provide a mobile radio system that does not require the excessive analog circuitry employed with analog systems and which is capable of higher capacity than the simple diversity associated with prior art digital systems.